Nylon can be dyed with acid or cationic dyes to give colored yarns which may be used in fabrics or carpets. Recently, yarn producers have begun incorporating colored pigments into nylon yarns to improve their resistance to degrading and fading in ultraviolet light, to give improved resistance to chemicals and noxious fumes and to give permanent coloration which is not removed by washing. While some pigments can be mixed easily into the nylon without adversely affecting the filament spinning operation, most pigments - and particularly organics - cause some difficulties while being mixed into the nylon or in subsequent melt-spinning and drawing operations. In general, organic pigments tend to cross-link nylon, change its viscosity, form spherulites which weaken the fibers, and cause increased draw tension and filament breaks.
Ultraviolet light degrades nylon, and the degradation can be accelerated by the presence of some pigments. To avoid this, copper in various forms is often added to the polymer. The amount of copper which is effective in preventing degradation of the polymer by ultraviolet light also causes poor spinning performance. The combination of pigment and copper is still worse.
European Patent Publication No. 0373655 ("Anton et al."), published Jun. 20, 1990, and incorporated herein by reference, discloses processes for making stain-resistant, pigment-colored fibers with acceptable levels of spinning performance. Those processes involve forming a random nylon copolymer made with up to 4.0 weight percent of a cationic dye additive such as 5-sulfoisophthalic acid or its salts, adding up to 4.5 weight percent of a pigment concentrate to the copolymer, and melt-spinning the pigment/polymer blend. Certain pigments, however, remain very difficult to spin even using the copolymers disclosed therein.
While Anton et al. is directed primarily at pigment-colored fibers useful in carpet applications, there is also a demand for pigmented fibers suitable for use in certain industrial applications such as for parachute fabrics, life-jackets, and industrial sewing thread where high tenacity is required. Here too, however, the presence of pigment has made it difficult to draw the fibers, and consequently insufficient orientation is obtained to achieve tenacity levels greater than 7.5 grams per denier, which is a level that can be attained for non-pigmented nylon yarns.
Ways of reducing the impact of such pigments on nylon spinning and drawing performance would permit the use of a wider selection of colored pigments, both organic and inorganic, would enable fiber producers to offer a complete range of styling colors while reducing product deficiencies and operating difficulties, and would allow for the production of high tenacity pigmented nylon fibers.